He warned that the drug could cause serious reactions in some people and even backfire in the case of an actual emergency, putting people past a two-week window of safe dosage. After that period of time, the drug can induce severe hypothyroidism, a condition that essentially shuts down thyroid function.

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In the event of a nuclear emergency, potassium iodide is most useful in protecting infants and children younger than 18, whose bodies are most vulnerable to the effects of radioactive iodine, according to the CDC.

Adults older than 40 are warned not to take KI unless contamination with a very large doses of radioactive iodine is expected. They're at the lowest risk for developing thyroid cancer after radiation exposure and at highest risk for having allergic reactions to KI.